1. Field of The Invention
This invention relates to novel silacyclohexane compounds and the preparation thereof. The invention also relates to liquid crystal compositions comprising the silacyclohexane compound or compounds, and to devices comprising the compositions.
2. Description of the Prior Art
The liquid crystal display devices utilize optical anisotropy and dielectric anisotropy of liquid crystal substances. Depending on the mode of display, different types of display systems are known including those of a twisted nematic type (TN type), a supertwisted nematic type (STN type), a super birefringence type (SBE type), a dynamic scattering type (DS type), a guest/host type, a type of deformation of aligned phase (DAP type), a polymer dispersion type (PD type), and an optical mode interference type (OMI type). The most popular display device is one which is based on the Schadt-Helfrich effect and has a twisted nematic structure.
Although the properties of the liquid crystal substances used in these liquid crystal devices depend, more or less, on the type of display, it is commonly required that the liquid crystal substances have a wide range of temperatures working as a liquid crystal and that they be stable against moisture, air, light, heat, electric field and the like. Moreover, the liquid crystal substances should desirably be low in viscosity and should ensure a short address time, a low threshold voltage and a high contrast in cells.
Liquid substances which can satisfy all the requirements have never been known when used singly. In practice, several to ten and several liquid compounds and/or latent liquid crystal compounds are used in the form of a mixture. To this end, it is important that constituent components be readily compatible with one another.
Among the various types of display modes, the twisted nematic mode which is driven with an active matrix (AM) using a TFT (thin film transistor) element array or an MIM (metal insulator metal) element array has now been in extensive use owing to its high image display qualities including high definition, high contrast and high response speed. This is because the DRAM making techniques in the industry of silicon semiconductors have been applied to and developed in the liquid crystal panel making techniques, thus leading to reduction of costs and promoting the advancement of the techniques.
The nematic liquid crystal substances employed in the active matrix liquid crystal devices (AM-LCD) should have not only the above-mentioned properties, but also a signal voltage holding or retaining characteristic which is essentially required for the drive system of the devices. This signal voltage holding characteristic indicates a degree of lowering of the signal voltage applied to TFT pixels including the liquid crystal within a given frame period. Accordingly, when the signal voltage does not drop at all wherein the voltage holding rate is 100%, the liquid crystal molecules are kept as they are in arrangement and any lowering of contrast does take place. The voltage holding characteristic is, more or less, influenced by the environment in which the liquid crystal is used. More particularly, in environments where the liquid crystal is exposed to light of high intensity such as in liquid crystal panel projectors or it is exposed to high temperatures such as in on-vehicle liquid crystal panels, the life characteristic is liable to be shortened.
From this point of view, compositions of liquid crystal compounds which have a positive value of dielectric anisotropy (.DELTA..epsilon.) and a core structure constituted of a cyclohexane ring or rings and a benzene ring or rings have been proposed for use in AM-LCD, for example, in Japanese Laid-open Patent Application Nos. 4-226589, 4-505477, 4-504880, 5-331464, 5-500680, 5-500681, 5-500682 and 6-501520.
Liquid crystal compounds which contain a fluoroalkoxy group as a polar group are known, including those compounds shown below: ##STR4## wherein R represents H or an alkyl or alkenyl group having from 1 to 15 carbon atoms, A.sup.1 and A.sup.1, respectively, represent hexylene or phenylene, Z.sup.1 and Z.sup.2, respectively, represent --CH.sub.2 CH.sub.2 --, L.sub.1 and L.sub.2, receptively, represent H or F, X represents H, F, Cl or C.sub.x H.sub.(2x+1-y)F.sub.y wherein x=1 to 5 and 0.ltoreq.y.ltoreq.2x+1, and m is 0, 1 or 2 as set out in Japanese Laid-open Patent Application No. 6-40988; ##STR5## wherein R, A.sup.1, Z.sup.1, A.sup.2, Z.sup.2, L.sub.1, L.sub.2 and m are, respectively, as defined above, L.sub.3 represents H or F, n is 0 or 1 as set out in Japanese Laid-open Patent Application No. 6-329573; and ##STR6## wherein R, A.sup.1, Z.sup.1 and m are, respectively, as defined above, A represents hexylene or phenylene, W represents O or COO, Q represents O or --CH.dbd.CH--, Y represents F, Cl, CF.sub.3, OCF.sub.3, CHF.sub.2, OCHF.sub.2, or OCH.sub.2 F, n is 0, 1 or 2, o is 0, 1 or 2, r is an integer of 1 or 2, and t is a value of 0 to 7 provided that n+o.gtoreq.2 as set out in Japanese Laid-open Patent Application No. 6-501959.
As liquid crystal display devices have now wider utility in various fields, the characteristic properties required for liquid crystal materials become severer. Especially, with portable liquid crystal devices whose electric power is based on batteries, reduced consumption power is essential. To this end, it is beneficial to lower a drive voltage of liquid crystal devices.
The increase in dielectric anisotropy (.DELTA..epsilon.) of liquid crystal materials enables one to permit lower voltage drive. However, the liquid crystal materials whose dielectric anisotropy is great are deficient in that their nematic-isotropic transition temperatures (T.sub.NI) are commonly low.
The lower drive voltage or lower threshold voltage results in a lower voltage holding rate. Thus, existing liquid crystal compositions are inevitable with a sacrifice of either a lower threshold voltage or a lower voltage holding rate to an extent.